1. Field of the Invention
The present invention relates to an image heating apparatus. In general, an image forming apparatus adopting an electrophotographic process generally has a fixing device (an image heating apparatus) that fuses and fixes a toner image through heat and pressure application as follows. That is, a recording material and a toner electrostatically carried on the recording material are nipped and transported at a pressure contact portion (nip portion) between heating means (roll, endless belt member, etc.) and pressure means (roll, endless belt member, etc.), which come into pressure contact with each other in a rotatable manner, the toner being formed of a resin, a magnetic material, a colorant, etc.
2. Related Background Art
FIG. 8 shows a structure of a belt-nip type fixing device as disclosed in JP H09-034291 A. Proposed in the publication is the device having: a rotatable heating roll 101 including a heat source; an endless belt 102 coming into pressure contact with the heating roll 101 and rotating in accordance with the rotation of the heating roll 101; a pressure roll 103a which is provided at the end on a downstream side of a contact nip area N between the heating roll 101 and the endless belt 102 and adapted to stretch the endless belt 102; and an assist pad 104 provided inside the endless belt 102 and adapted to pressurize the endless belt 102 against the heating roll 101. Here, the endless belt 102 is rotatably stretched over the pressure roll 103a and plural support rolls 103b and 103c. 
In such a belt-nip type fixing device, the pressure roll 103a enables a recording material 105 to peel off at an outlet of the contact nip area N, i.e., self-stripping, while requiring no stripping claw even in the case of fixing the toner image in a large toner amount onto the recording material in a thin paper form, which is low in stiffness. That is, in general, the heating roll 101 has an elastic layer constituting its surface. When the pressure roll 103a comes into pressure contact with the elastic layer, the surface of the heating roll 101 elastically deforms at a pressure contact area between the heating roll 101 and the pressure roll 103a, and the recording material 105 passing between the heating roll 101 and the pressure roll 103a is guided in such a direction as to peel off from the heating roll 101.
However, if the pressure of the pressure roll 103a is excessively increased for improving a self-stripping performance, there occurs a phenomenon that an unfixed toner image 106 on the recording material 105 is offset. This phenomenon results from the fact that the high pressure of the pressure roll 103a causes the elastic layer on the heating roll 101 to excessively deform, leading to an increased moving speed of the recording material 105. To cope with the image offset, the pressure pad 104 is provided. In other words, the pressure pad 104 is used to pressurize the heating roll 101 in a wide range on an upstream side of the pressure roll 103a within the contact nip area N. Thus, it is unnecessary to excessively increase the pressure applied to the heating roll 101 from the pressure roll 103a, thereby preventing the image offset.
If an electrophotograph having a glossiness equivalent to printed matter is demanded, the gloss of a fixed toner image needs to be increased. As understood from the above, the fixing device adaptable to glosses in a wide range is desired. To attain this, a method of changing a surface temperature of the heating roll or changing a time for passing the nip may be adopted. However, in the method of changing the surface temperature of the heating roll, since a heat quantity of the heating roll is large, when a rate at which the temperature is changed is large, the image formation cannot be carried out until the temperature is changed to the target temperature, which undesirably results in a remarkable decrease in productivity. On the other hand, the method of changing the time for passing the nip involves the following problems.
The time for passing the nip is defined as a numerical value calculated from the equation of nip width (mm)/sheet speed (mm/s), which corresponds to a heating time of the recording material and the toner. FIG. 7 shows a relationship between a toner surface gloss and the time for passing the nip with a uniform area pressure in the nip. In this experiment, the time for passing the nip is changed by changing a process speed, but the same effects can be obtained also by changing the nip width. A controlled temperature of the heating roll is set constant at 190° C. during the experiment. With a relatively high pressure of 3 kg/cm2, the gloss increases up to 90% in accordance with the increase of the time for passing the nip. On the other hand, with a relatively low pressure of 1.5 kg/cm2, the gloss increases up to 50% but when the gloss exceeds 50%, the gloss experiences a downturn in increase. With a much lower pressure of 0.5 kg/cm2, when the gross is not less than 20 to 30%, the gloss tends to experience a downturn in increase.
A glossiness detecting method is for measuring a glossiness based on JIS Z8741. That is, as for a measurement method, a luminous flux is made incident on the recording material surface at a stipulated angle of incidence with a stipulated angle of aperture to thereby measure the luminous flux reflected in a mirror reflection direction with the stipulated angle of aperture by a photo-detector 1090. In FIG. 9, the luminous flux irradiated from a light source 1080 passes through a lens and enters a recording material P at an incident angle θ. The luminous flux reflected in the mirror reflection direction is detected by the photo-detector 1090 through a lens 1100. The detection is performed on the surface glossiness with the incident angle θ set to 60°.
On the other hand, a fixability that the toner image is fixed onto the recording material is mainly largely affected by the heat quantity for fusing the toner, if a given pressure is applied. This imposes a limitation on a lower limit to which the time for passing the nip can be minimized. As a result, in the method of changing the time for passing the nip, a gloss change range is narrow.
However, as apparent from FIG. 7, the gloss increases if the pressure is increased.
This is supposedly achieved on the following two grounds.
First, in the case of the toner containing oil as a mold release agent, the pressure at the nip portion is increased, so that the oil easily leaks from the toner, with the result that the oil adheres to the heating roll to improve a mold release property of the heating roll to the toner. Therefore, it is possible to form a smooth fixed toner image. Second, in the case of the heating roll having an elasticity, the surface of the heating roll is pulled due to the pressure at the nip portion when the pressure is high, thereby increasing a smoothness of the surface of the heating roll within the nip, so that the smoothness of the fixed toner image is increased.
Further, while the minimum pressure is set, the time for passing the nip is secured, so that the fixability can be assured.
In this way, the gloss control can be made in a wide range by changing the pressure without changing the time for passing the nip.
In the structure of FIG. 8, when the pressure is changed to the plural different pressures using one planer pressure member, the pressure change range should be made narrow. In particular, with one planer pressure member, any error is likely to occur with respect to a variation and thus, the target gloss cannot be obtained. Therefore, the pressure change range should be made wide.